In the art of paging persons by means of portable radio receivers carried on the person that are responsive to an assigned carrier frequency, it is known to modulate the carrier with a sequence of calling frequencies, or "tones", for the purpose of signalling to subscribers with unique combinations of tones, for example, to address a particular subscriber, or to broadcast a particular message. A known two-tone paging system uses two tones selected in a coded sequence from an array of 70 available tones that are spaced a constant percentage apart from each other in frequency. This system affords 4,900 possible calling combinations, (that is, 70.sup.2 = 4,900), some of which may not be useful. A known five-tone system uses five tones selected from an array of 11 available tones, to provide 100,000 possible calling combinations of paging tones on a single carrier frequency. In such a system, five tones are sent out (i.e.: modulate the carrier) in a coded sequence, but the number of tones used in a given coded sequence is not critical. In any system, each individual receiver is responsive to a selected one or few of all the possible useful codes. The present invention is described in connection with a multi-tone signalling system, to illustrate a presently-preferred embodiment of it as applicable to paging systems using space-transmitted (radio) wave energy, as well as wired paging systems in which the receivers may be in fixed locations.
In a five tone paging system, it is known to use 10 or more tone-frequencies which are each separated from the adjacent tones by a constant frequency difference, .DELTA.f. This use of constant .DELTA.f yields tones that follow each other in equal steps, n.DELTA.f, where n is an integer. In practice, the equal steps are related to a particular base or starting tone frequency, f.sub.o, such as 459Hz. Thus, if f.sub.o = 459Hz, and .DELTA.f = 141Hz, the following series of tone-frequencies will result ("Hz" being omitted: 459, 600, 741, 882, 1023, 1164, 1305, 1446, 1587, 1728, 1869, etc. None of these frequencies is harmonically related to any of the others. In a given five-tone page, each tone lasts 33 milliseconds, and there is a 35 millisecond gap between pages, resulting in (5 .times. 33)+35 = 200 milliseconds for a complete page. Such a 5-tone paging system allows five pages per second.
In order to detect tone signals in an observation time interval independent of the tone frequency, it is known also to use a tone filter which has the same rise time for every tone in the system. This property of constant rise time with respect to frequency is a property of the constant bandwidth filter. The combination of constant .DELTA.f, with constant bandwidth in the tone filters, provides a multi-tone paging system which makes economical use of time. Such a system has also the property that the Q of the tone filter changes with respect to tone frequency, thereby enabling the adjacent tone rejection to be maintained relatively fixed. With fixed adjacent tone rejection, the adjacent bands (in the tone filter) can be evenly spaced (i.e.: .DELTA.f = a constant).
The requirement for a constant bandwidth filter is satisfied by a filter known as the "Biquad" Active Filter, which is described in articles by Lee C. Thomas in IEEE Transactions on Circuit Theory, Vol. CT-18, No. 3, May, 1971, pages 350 to 361. The articles are entitled "The Biquad Part I -- Some Practical Design Considerations" (pp. 350-357); and "The Biquad: Part II -- A multipurpose Active Filtering System" (pp. 358-361). FIG. 1 of the latter article (Part II) shows a structure that realizes a general biquadratic voltage transfer function the coefficients of which are functions of resistance values.
It is known to use a network of "2n" switchable resistors in an Active Filter to achieve "n" tones from the filter. The resistors required are of a high-precision type. Since in practice each resistor is switched with a field-effect transistor (FET) or equivalent semiconductor device, such a use requires that "2n" switching devices also be present. For a 16-tone filter (i.e.: a filter having 16 pass-bands) according to the known prior art, therefore, it is required to provide 32 high precision resistors and 32 FET switching devices.